A typical organic light emitting diode (OLED) device, as illustrated in FIG. 1, is an electronic device including a multilayered structure 100 including an organic layer 104 disposed between an anode 106 and a cathode 108. The organic layer 104 typically comprises a hole transport layer 112 and an electron transport layer 114 and may further include a luminescent layer 110 disposed between the hole transport 112 and electron transport layers 114. The substrate 102 typically comprises glass but may be formed from another light, flexible, transparent material such as a plastic. In a typical OLED device, the anode layer 106 comprises indium tin oxide (ITO) and the cathode layer 108 is metallic containing metallic materials such as silver, gold, calcium or magnesium.
Providing an electrical charge to the electrodes induces formation of charged molecules within the hole transport and electron transport layers. These charged molecules recombine in the luminescent layer to produce light by electroluminescence.
A primary application of OLED device technology is in flat panel displays as an alternative to liquid crystal displays (LCDs). OLED displays offer a number of advantages over LCD technology including that they are flexible and provide a wider viewing angle and superior colour resolution. Since OLED devices are self-luminous, they do not require backlighting and therefore consume significantly less energy than LCDs. Further more, OLED displays are potentially faster, lighter and brighter than LCDs.
An OLED display comprises an array of pixels deposited on a substrate in a matrix of rows and columns. Each pixel consists of a light emitting diode formed at the intersection of each column and row. There are two types of OLED displays, active matrix and passive matrix. In a passive matrix display, each pixel is illuminated by applying an electrical charge to the row and column that intersect to define that pixel. In an active matrix display, each light emitting diode is associated with a thin film transistor, which controls the amount of current flowing through the OLED.
Passive matrix OLED devices are manufactured by depositing the various layers of the multilayered OLED device onto a substrate and patterning the electrode layers into a plurality of discrete conductive elements. Conventional patterning processes include photolithographic and chemical etching techniques. However, OLED materials and device structures tend to be incompatible with conventional patterning techniques. Exposure to oxygen, water vapour and chemicals in the process of patterning the cathode layer may cause degradation of the underlying organic layers. Furthermore, conventional patterning techniques create additional problems including structural defects or contamination of the substrate causing the OLED to be susceptible to electrical shorts.
Laser ablation techniques involve the removal of portions of layers of electrically conductive and insulating materials in accordance with a predetermined pattern by exposing the selected areas to a laser beam. These techniques have demonstrated the potential to offer faster and more accurate production of OLED devices. It is possible to directly pattern conductive films by laser without the need for multi-step processes involved in photolithography such as mask preparation, exposure, developing and etching.
Patterning of the cathode using existing techniques typically results in a series of “mushroom” structures as shown in FIG. 2. The passive matrix OLED device 200 shows the configuration of the “mushroom” structures 210 formed during a conventional patterning process. The OLED device 200 comprises an indium-tin oxide anode 206 deposited on a glass substrate 202 and an organic light-emitting layer 204 disposed between the anode layer 206 and the cathode layer 208. The “mushroom” structures 210 are configured such that they generally do not have parallel walls. That is, the base 212 of the “mushroom” 210, has a smaller cross section than the upper portion 214 of the “mushroom” 210. In addition, the cathode layer 208 may be encapsulated with an encapsulating layer 216 to protect the metallic cathode 208.
The configuration described prevents optimum performance from being achieved. Small variations in the slant of the “mushroom” wall can be the difference between the device being, or not being, susceptible to electrical shorts. Another cause of electrical shorts is an excessively sharp slant causing the “mushroom” to collapse during the baking process.
Furthermore, the methods described usually leave the organic layers and the cathode layer unpatterned, leading to cross talk between neighbouring pixels. Attempts have been made to combat this problem by replacing the low resistivity hole transport layer with a high resistivity layer. However, this solution compromises the conductivity of the organic layers thereby reducing overall performance and efficiency of the device. Moreover, the methods currently employed for patterning of OLED devices are considered to be too complex, slow and costly for high volume manufacturing.